The instant invention relates to solders and more particularly to a novel solder alloy which is particularly effective for use in automotive radiator applications.
Generally, when a solder is used in an automotive radiator application, it is exposed to a relatively severe environment wherein it is subjected to fluctuating temperatures, mechanical vibration, immersion in a water-based fluid, and electrical connection to other dissimilar metals. Hence, in order for a solder to be effective for use in an automotive radiator application, it must have a relatively high degree of mechanical strength, and it must be highly resistant to corrosion. In addition, such a solder must be readily solderable to brass and other metals utilized in radiator constructions, and in order for it to be practical for use in the highly competitive automotive industry, it must be reasonably priced.
For many years, "30/70" solders comprising 30% tin and 70% lead were used extensively for automotive radiators. However, while these solders were highly resistant to corrosion, they had relatively poor high temperature mechanical properties, and they were relatively expensive. As a result, more recently a number of high lead solder alloys having substantially reduced tin contents have been developed, such as "5/95" alloys comprising 5% tin and 95% lead, and Modine alloys comprising 2.5% tin, 0.5% silver, and 97% lead, and these high lead solders have largely replaced the "30/70" solders in automotive radiator applications. However, while these high lead solder alloys have proven to be substantially less expensive than the earlier "30/70" solder alloys, and to also have substantially better mechanical properties than the "30/70" alloys, they have been found to be substantially more succeptible to corrosion over prolonged periods of use.
Generally, most of the corrosion which occurs in a solder alloy when it is used in an automotive radiator application is caused by galvanic action between the solder and the other metal components of the radiator which are in contact with the solder. Galvanic corrosion is caused when two dissimilar metals are placed in intimate contact with each other and immersed in an electrolyte. Under these conditions, an electrical potential is produced between the two metals and the less noble metal, i.e. the more active metal, becomes an anode and corrodes while the more noble metal becomes a cathode and remains substantially uncorroded. Since the metal components of radiators are normally made of metals, such as brass, which are more noble than most solders, the solder in a radiator normally becomes the anode in a galvanic reaction and it is therefore succeptible to corrosion. It has been found, however, that if the exposed surfaces of the solder in a radiator are protected by a corrosion resistant film, the galvanic action between the solder and the other metal components of a radiator can be substantially reduced. Further, it has been found that if a sufficient amount of tin is present in a solder alloy, the tin naturally forms a protective or passivating oxide layer on the surfaces of the solder, and this oxide layer, being substantially insoluble in water and most antifreezes, can be sufficient to protect the solder from the galvanic action which occurs in an automotive radiator. However, since a tin oxide layer of this type is inherently more brittle than the solder on which it is formed, any ductile deformation of the solder (via creep or plastic elongation) will result in mechanical damage to the film and cause increased corrosion in the solder. Hence, in order for a solder to be resistant to corrosion, it must not only be capable of producing a passivating surface film, but it also must have sufficient mechanical strength to make it resistant to creep deformation in order to avoid or at least minimize damage to the protective film.
Generally, all lead based alloys have very low yield points, and they are subject to some plastic deformation under even minimal loading conditions. Lead based alloys, such as solders, are particularly prone to creep deformation, which is a time-dependent plastic deformation of the material under load. Although creep deformation is usually only a matter of concern when the operating temperature to which a solder alloy is exposed exceeds one-half of the absolute melting temperature of the alloy, the solders used in automotive radiators are frequently exposed to conditions of this nature. Creep in lead based alloys is due both to intergranular sliding and to single grain elongation and therefore, in order to minimize creep deformation, both intergranular sliding and single grain elongation must be minimized.
The instant invention provides a solder alloy which has both high resistance to galvanic corrosion and also high resistance to creep deformation, making it highly effective for use in automotive radiator applications. The solder alloy of the instant invention consists essentially of 87.5%-96.5% lead, 3%-10% tin, 0.5%-2.0% antimony, and 0%-0.5% silver, the preferred alloy being approximatley 93.65% lead, 5% tin, 1.2% antimony, and 0.25% silver. It has been found that a solder alloy having these components in the specified concentrations is capable of producing an effective corrosion resistant passivating layer of tin oxide on the surfaces thereof when utilized in an automotive radiator environment, and it also exhibits sufficient resistance to creep deformation to minimize the damage to the passivating layer formed on the surfaces thereof when it is used in an automotive radiator application. It has also been found that a solder formed in accordance with the instant invention has effective solderability characteristics, and that it can be manufactured at a cost which makes it competitive with other solders which have been heretofore available for similar applications.
Accordingly, it is a primary object of the instant invention to provide an improved solder alloy for automotive radiator applications.
Another object of the instant invention is to provide a solder alloy which is highly resistant to corrosion from galvanic action, and also highly resistant to creep deformation.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds.